178 INTRODUCTION TO GENERAL SCIENCE 



to act. It must be remembered, however, that the thickness 

 of the earth's crust is slight compared with its Diameter, and 

 that the interior of the earth is hot, and would, under ordinary 

 conditions, exist in a liquid form, but does not at present, on 

 account of the tremendous pressure which it bears. In deep 

 mines the temperature is so high that it is impossible to work 

 continuously more than a short time. See Section 48, The 

 Heat of the Earth. 



References : 



1. 1205 : 17-21. Mantle Rock and the Soil. 



2. 1304 : 16-22. The Solid Earth. 



a. 1201 : 19-22. How the Rocky Crust was Hidden. 



b. 1203 : 13-19. The Decay of Rocks and the Causes. 



c. 1209:16-17. Weathering. 



d. 1302 : 29-31. The Earth Crust ; Mantle-Rock. 



e. 1303 : 134-138. Activities and the Wasting of the Land. 

 /. 1305 : 180-182. The Structure of the Earth. 



g. 1309:75-76. Relief of the Land. 

 h. 1311 : 11-13. Continuous Change. 



126. CHANGES IN THE SURFACE OF THE EARTH 



By " changes " is meant all those modifications of the sur- 

 face which have taken place since the earth became cold 

 enough to allow water to fall upon it, as well as the modifica- 

 tions which are taking place at the present time. The large 

 changes, such as the formation of rocks, and the building of 

 mountains, will be taken up under those headings. Now we 

 are to consider those alterations, or modifications, which affect 

 and have affected the surface itself; by " surface " is meant 

 only the few upper feet of the earth. Refer to Section 139, How 

 Mountains are Made, and to Section 133-137, which -treat of 

 rocks and their formation. 



